The prior art, for example U.S. Pat. No. 3,946,175 A, discloses a temperature-compensated, pressure-gradient-controlled pressure switch. Pressure switches of this type are used in pressurised containers in order to keep the triggering pressure or switching pressure as close as possible to the current operating pressure. If pressurised containers are exposed to large temperature variations, which may be the case for example in fire extinguishers on an aircraft, the operating pressure changes considerably depending on how full it is.
In a pressure switch of this type, it always has to be ensured that it is not triggered unintentionally. In order to ensure this in non-temperature-compensated pressure switches, they are usually designed such that a relatively large ratio is provided between operating pressure and triggering pressure; in non-temperature-compensated pressure switches, this pressure ratio is usually between 2 and 4. If, therefore, the operating pressure is 8 bar, at a pressure ratio of 4 the pressure switch only switches in the event of a drop in pressure to 2 bar.
The pressure switch having temperature compensation known from U.S. Pat. No. 3,946,175 A comprises a secondary pressure chamber, which is filled with the same contents and mass ratios as the primary pressure chamber, such that the thermodynamic behaviour of the primary pressure chamber is identical to that of the secondary pressure chamber, with the exception of the lower volume and the thus lower thermal capacity. The secondary pressure chamber is hermetically separated from the primary pressure chamber by a flexible dividing element. A pressure difference between the primary pressure chamber and the secondary pressure chamber can thus be converted into a translational movement by the flexible dividing element. This movement is then transferred to a carriage, which is provided with a magnet. Owing to the movement of the magnet, a switching mechanism, in this case a Reed switch, can be then actuated.
Furthermore, in U.S. Pat. No. 3,946,175 A the secondary pressure chamber is thermally coupled to the primary pressure chamber so as to ensure that a temperature variation in the primary pressure chamber brings about an approximately identical pressure change in the primary and secondary chamber. The flexible dividing element thus remains substantially in its starting position, such that the carriage is slightly moved thereby in any case. The switching mechanism actuatable by the magnet is not triggered in this state, however.
Reference can thus be made to a virtually temperature-independent pressure switch, which is why the pressure ratio between the operating pressure and the triggering pressure is usually between 1.3 and 1.6. The switching mechanism is therefore actuated even in the event of small drops in pressure if a drop in pressure is caused by a leak in the primary pressurised container, for example. A drop in pressure in the primary pressure chamber due to a drop in temperature in the surroundings does not trigger the switching mechanism, however.
Temperature-compensated, pressure-gradient-controlled pressure switches of this type are used in fire extinguishers in aircraft, for example. In this case, it has to be ensured at regular intervals that the installed pressure switch is still functional.
In the test methods previously known from the prior art, it was necessary to completely empty the primary pressurised container in order to test the pressure switch. However, opening, emptying and refilling the primary pressurised container is complex and expensive. In addition, in this method climate-damaging gases may be emitted, which are released by the filling and emptying process and by the recycling of the contents.
Lastly, a test method of this type has the drawback that the primary pressurised container can be damaged by being taken apart and then welded; a test method of this type cannot be repeated an unlimited number of times either; see BEATTIE, A. G. An Acoustic Emission Test for Aircraft Halon 1301 Fire Extinguisher Bottles, Federal Aviation Administration, Report No. DOT/FAA/AR-97/9, 1998.
Furthermore, U.S. Pat. No. 3,946,175 A discloses a test method in which a switching mechanism, which in this case is formed by the Reed switch, is triggered by a strong external magnetic field. However, only the electronic circuit can be tested using this method. Statements therefore cannot be made on the triggering pressure and the mechanical switching capability of the pressure switch, i.e. on the error-free freedom of movement of the carriage, for example.
The problem addressed by the invention is to provide a method and a corresponding device for more reliably testing the mechanical and electronic functionality of a temperature-compensated, pressure-gradient-controlled pressure switch.
The problem is solved by the features of the independent claims.
According to the basic concept of the invention, a method for testing a temperature-compensated, pressure-gradient-controlled pressure switch, which is pneumatically associated with a primary pressure chamber and comprises a secondary pressure chamber for temperature compensation, is proposed, wherein, by using at least one temperature-control apparatus between the primary pressure chamber and the secondary pressure chamber, a temperature gradient is set which is sufficiently great to produce a pressure gradient between the primary pressure chamber and the secondary pressure chamber that corresponds at least to the triggering pressure gradient of the pressure switch.
According to the invention, selectively heating and/or cooling the primary and/or secondary pressure chamber means that it is made possible for the pressure switch to be triggered due to a temperature gradient being produced. The invention has recognised that temperature gradients of approx. 30 K are usually sufficient to bring about a triggering pressure in common temperature-compensated pressure switches. Depending on the contents, however, the triggering pressure may be reached at smaller or also at greater pressure gradients. Here, the triggering pressure gradient is the relative pressure between the primary pressurised container and an operating pressure at which the switching mechanism in the pressure switch changes the switching state.
Furthermore, the required temperature gradient can be reached within a relatively short time of approx. 30 minutes by the method according to the invention, and the switch can be triggered entirely without any intervention in the system, i.e. without the primary pressure chamber being emptied and refilled.
In addition, the method can carry out both an electronic and a mechanical test of the pressure switch. Preferably, a control apparatus is also provided, which automatically stops the temperature gradient from increasing further when the triggering pressure is reached, so that unnecessary cooling and/or heating can be prevented. The method thus allows efficient, effective and environmentally friendly testing of temperature-compensated pressure switches.
It is proposed that the secondary pressure chamber is heated or cooled by the first temperature-control apparatus. Preferably, the temperature of the primary pressure chamber, i.e. of the chamber filled with a filling medium, is kept constant. The required temperature gradient is therefore preferably reached solely by cooling or heating the secondary pressure chamber. Preferably, the secondary pressure chamber is heated such that the filling medium in the primary pressure chamber is heated considerably more slowly and thus in a delayed manner due to the heating of the secondary pressure chamber, in particular because of the greater mass. The desired temperature gradient thus arises automatically between the primary pressure chamber and the secondary pressure chamber after a certain length of time owing to heating of the secondary pressure chamber.
Preferably, an active cooling or heating element is thermally connected to the primary pressure chamber such that the thermal effects of the first temperature-control apparatus on the primary pressure chamber are compensated for at least in part. Preferably, the active cooling or heating element is arranged to be in direct contact with a surrounding wall forming the primary pressure chamber. Preferably, the active cooling or heating element is also arranged in the spatial proximity of the first temperature-control apparatus, i.e. preferably less than half the distance of the pressure switch from the furthest point on the primary pressure chamber, more preferably less than a third of this distance and particularly preferably less than a tenth of this distance.
An active cooling or heating element is advantageous in particular if sufficient compensation of the temperature effect of the temperature-control apparatus on the primary pressure chamber by the thermal insulation apparatus is not possible.
Also preferably, the primary pressure chamber is cooled or heated by the second temperature-control apparatus. Preferably, cooling is carried out by the second temperature-control apparatus when heating of the secondary pressure chamber is carried out by the first temperature-control apparatus, and vice versa. Owing to the heating, for example of the secondary pressure chamber, and to the cooling of the primary pressure chamber, the required temperature gradient can be reached more rapidly. Preferably, the secondary pressure chamber is heated by the first temperature-control apparatus at the same time as the primary pressure chamber is cooled by the second temperature-control apparatus.
Preferably, the primary pressure chamber is oriented such that a liquid phase of a filling medium contained therein, which phase is formed by CBrF3 for example, has as small as possible a contact surface with the pressure switch, and more preferably there is no contact surface at all. This may for example be achieved by the pressure switch being brought into the highest possible position while the method is being carried out. The invention has recognised that, due to this orientation, the gas phase which preferably largely or completely surrounds the pressure switch assumes the function of an insulating medium. The heat transfer from the pressure switch to the primary pressure chamber can thus be reduced, and therefore the temperature gradient becomes greater overall.
According to the invention, a test device for testing a temperature-compensated, pressure-gradient-controlled pressure switch is proposed, which is pneumatically associated with a primary pressure chamber and comprises a secondary pressure chamber for temperature compensation, wherein a temperature-control apparatus is provided by means of which a temperature gradient between the primary pressure chamber and the secondary pressure chamber can be set, and the pressure switch can be actuated by a pressure gradient caused by the temperature gradient. According to the invention, the test device provides the advantage that the triggering pressure can be generated in a simple manner by producing a temperature gradient between the primary pressure chamber and the secondary pressure chamber.
Preferably, the test device is designed to set a temperature difference of at least 10 K between the primary pressure chamber and the secondary pressure chamber, more preferably of at least 20 K and particularly preferably of greater than 30 K. These temperature gradients are required for building up a sufficient pressure gradient that corresponds to the triggering pressure of the pressure switch.
Also preferably, a first temperature-control apparatus is designed to cool or heat a surrounding wall of the secondary pressure chamber in a targeted manner. Preferably, the temperature-control apparatus is flexibly and adaptably designed, so that there is as large as possible a contact surface between the first temperature-control apparatus and the pressure switch. Within the meaning of this invention, targeted heating is understood to mean that greater than 50% of the thermal energy output by the temperature-control apparatus can be transferred to the pressure switch, more preferably greater than 70% and particularly preferably greater than 90%. Preferably, targeted heating is also understood to mean geometric adaptation of the temperature-control apparatus to the geometry of the object to be heated. The same applies to the term targeted cooling, where at least 50% of the thermal energy absorbed by the temperature-control apparatus is absorbed by the object to be cooled, more preferably greater than 70% and particularly preferably greater than 90%.
Preferably, the first temperature-control apparatus is formed by a silicone heating mat, which preferably has a power of between 10 and 30 W, more preferably of less than 20 W. Owing to the flexible design of the temperature-control apparatus, the best possible heat transfer from the first temperature-control apparatus, via the surrounding wall forming the secondary pressure chamber into the secondary pressure chamber itself, is made possible. Because the pressure switch usually projects largely into the primary pressure chamber, said switch is only accessible to a limited extent. Preferably, the first temperature-control apparatus is therefore designed to be attached to the part of the pressure switch positioned on the outside of the primary pressure chamber. Preferably, the temperature-control apparatus is attached to the pressure switch by an attachment element, for example by a tensioning element, a clamp or a screw clamp.
It is also proposed that a second temperature-control apparatus is designed to cool or heat a surrounding wall of the primary pressure chamber in a targeted manner. Since the first temperature-control apparatus is preferably designed to heat the secondary pressure chamber, the second temperature-control apparatus is preferably designed to cool the primary pressure chamber. In this case, it has proven advantageous for the second temperature-control apparatus to form a contact surface with the surrounding wall of the primary pressure chamber. Advantageously, the contact surface is arranged on the side of the primary pressure chamber opposite the first temperature-control apparatus. The first, heating temperature-control apparatus is therefore arranged as far away as possible from the second, cooling temperature-control apparatus. More preferably, the second temperature-control apparatus is arranged such that the liquid phase of a filling medium within the primary pressure chamber covers the surface on the inside of the surrounding wall, which is directly opposite the second temperature-control apparatus. It is thus ensured that heat is transferred from the second temperature-control apparatus through the surrounding wall to the liquid phase of the filling medium, as a result of which the primary pressure chamber can be cooled more efficiently.
In addition, a thermal insulation apparatus is proposed which surrounds the outer surface of the surrounding wall. This is particularly advantageous if a second temperature-control apparatus only surrounds part of the surrounding wall and therefore the rest of the surrounding wall is heated by ambient heat. This negative effect can be prevented by the insulation effect.
Lastly, a first sensor apparatus is provided to determine the temperature of a surrounding wall of the secondary pressure chamber and/or a second sensor apparatus is provided to determine the temperature of a surrounding wall of the primary pressure chamber. The temperature sensors can be used to produce input data for a control apparatus. Therefore, for example, the cooling and/or heating power can be reduced if an excessively high or excessively low temperature is detected.
It is advantageous for a detection apparatus to be provided to determine the fill level of the primary pressure chamber. Preferably, the detection apparatus may be formed by an ultrasonic measuring appliance or by a mass determination apparatus. Knowledge of the fill level within the primary pressure chamber together with the temperature data at various points in the primary pressurised container makes it possible to calculate the temperature distribution within the primary pressurised container on the basis of a thermodynamic fluid model. On the basis of this temperature distribution, a pressure ratio can in turn be calculated that makes it possible to draw a conclusion on the precise triggering pressure gradient of the pressure switch.
It is also advantageous for a continuity tester to be provided to determine the switching state of the pressure switch. A multimeter that is connected to the actual switching mechanism of the pressure switch by an electronic line may be used as continuity tester, for example. If said switch is opened or closed, this can be detected by the continuity tester. Preferably, the continuity tester is connected to the control apparatus by means of signals, so that the test process can be influenced depending on the switch position. Therefore, for example, the secondary pressure chamber can only be heated until the continuity tester detects that the switching state has changed. Unnecessary thermal loading is thus prevented.